Rework scenario to allow running them across several images

This PR does a lot updates to initially cool idea:
 * Use behave's Scenario Outlines feature to run the same scenario with various data (here: different images - x86_64 and i386)
 * Join existing scenarios in one:
   Having three scenarios depending on each other is weird - we can't login if bootloader/kernel has failed to startup
 * Set a default timeout for each step and allow overriding it: e.g. ```systemd to be running``` will use a default timeout and ```systemd to be running  to appear in 480 seconds at most```